In particular, the invention relates to a mirror for use in a so-called adaptive optical system, wherein disturbances of wavefronts of the radiation reflected by the mirror are compensated for, for example, by operation of actuators which act upon the mirror or other components of the optical system, said disturbances being caused by deformations of a mirror surface of the mirror or other components of the optical system. Such deformations of the mirror surface may, for example, be caused by temperature influences which change over time or by mechanical influences.
In order to detect changes in a deformation state of a mirror, usually extensometers are employed which are adhered to a side of a mirror substrate opposite to the mirror surface. Measuring signals obtained from the extensometers are approximately representative of the deformation state of the mirror. However, the exact deformation state of the mirror cannot be detected with a sufficiently high accuracy as it would be desired, because the connection between the extensometer and the mirror is inadequate since the adhesive layer is disposed therebetween. Due to the adhesive layer, the extensometer may creep relative to the mirror substrate which results into measurement deviations, among others, in the form of hysteresis.